The present invention relates to a vehicle suspension system, and more particularly to a tandem axle suspension system in which each articulation mode is independently adjustable.
A variety of known suspension systems are available for passenger and heavy duty vehicles. One type of suspension system for heavy duty vehicles is air-based and includes the ability to adjust particular stiffness characteristics. Articulation of a tandem suspension is commonly separated into four distinct modes known as jounce, pitch, roll and warp. Stiffness in each of the modes depends primarily on spring forces developed through articulation at each corner. A stiffness adjustment of one mode typically effects the stiffness in one or more of the other modes.
With commercial of heavy-duty vehicles, it is advantageous to have low jounce stiffness that results in a soft ride. It is also advantageous to have a very high roll stiffness to counter high centers of gravity.
Warp stiffness is particularly relative to a tandem axle. High warp stiffness directly effects single wheel lift such as that caused when the suspension system strikes a curb. When a conventional tandem suspension system strikes a curb, the entire suspension system is twisted. As conventional suspension systems are relatively stiff in warp, rather large forces are transmitted through the entire suspension system even though only a single wheel contacts the curb. Multiple curb strikes may eventually disadvantageously affect the suspension system and may cause undesirable wear thereof.
Pitch stiffness in a tandem suspension is almost unnecessary as the sprung mass pitching forces are reacted as jounce forces on the tandem suspension as well as the vehicle""s other suspension systems.
When loading a trailer, it is common to drive a lift truck onto the trailer bed. Because the lift truck is typically very heavy, a large load is rapidly imposed on the suspension system, which cannot react quick enough to avoid having the trailer bed drop suddenly as a result of the weight of the lift truck. Conversely, when the lift truck is driven off the trailer bed, the trailer may bounce upward in response to the force that is provided by the air-based suspension system.
Loading a trailer with a lift truck poses at least two problems. First, the repetitive downward and upward motion provides undesirable wear on the suspension system. Second, when the trailer is at a docking station, it is necessary to maintain the truck bed adjacent the loading dock to enable workers to easily move between the truck bed and the loading dock. The wheels of the trailer are therefore commonly locked once the trailer is properly positioned. However, conventional suspension systems typically move in an arc. Thus, when the lift truck drives onto the trailer, the trailer bed drops, and the suspension moves through its arc. Because the wheels are locked, the trailer moves forward and away from the dock, in a motion commonly referred to as xe2x80x9cdock walk.xe2x80x9d Repetitive dock walk may cause the trailer to move a distance away from the docking station which may create loading and unloading difficulties.
Accordingly, it is desirable to provide a suspension system that provides for stiffness in each articulation mode to be independently adjusted to achieve the goals discussed above.
The suspension system according to the present invention provides a pair of walking beams mounted parallel to a longitudinal main frame of a vehicle subframe to support tandem axles. One or more airbags are attached between the walking beams and the subframe. The walking beams are mounted to lower outboard brackets extending from each axle. A spherical joint located at each longitudinal end of each walking beam engages the lower outboard brackets.
Each walking beam is constrained against fore-aft movement through a pair of watts linkages separated by a lateral torsion tube. Each walking beam is connected to its respective watts linkage through a revolute joint. A pair of tie-rods are attached through spherical joints between each watts linkage and a support which extends perpendicularly from the subframe. Each axle is further restrained by a V-rod attached between each axle and the subframe. The V-rod is mounted to a respective axle at the single upper bracket through a spherical joint.
The present invention provides for stiffness in jounce, roll, pitch and warp to be independently adjusted. In jounce, the stiffness is controlled solely by the airbags. In roll, the airbags are augmented by the stiffness of the torsion tube between the watts linkages. In pitch, the tandem axles are movable to xe2x80x9cequalizexe2x80x9d with little resistance. In warp, each axle is in opposing roll and free to move on the spherical joints with minimal resistance neither transmitting forces through the air springs nor the torsion tube.
The present invention provides stiffness in jounce, and roll, but minimal stiffness in the warp mode which significantly reduces single wheel lift force. The semi-balanced suspension system also provides minimal or no stiffness in pitch. A vehicle or trailer that is supported at either end by a suspension system according to the present has no need of pitch stiffness at either end as the trailer will pitch as a body in proportion to the jounce stiffness at either end.
Dock walk is practically eliminated because there is no swing-arm effect in the kinematics of the tandem suspension.